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Date:   Wed, 11 Nov 2020 18:26:20 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>, mingo@redhat.com,
        acme@kernel.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
        jolsa@redhat.com, eranian@google.com, ak@linux.intel.com,
        dave.hansen@intel.com, kirill.shutemov@linux.intel.com,
        benh@kernel.crashing.org, paulus@samba.org,
        David Miller <davem@davemloft.net>, vbabka@suse.cz
Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
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On Wed, Nov 11, 2020 at 06:22:53PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 11, 2020 at 04:38:48PM +0000, Matthew Wilcox wrote:
> > 	if (pud_leaf(pud))
> > 		return PUD_SIZE;
> 
> But that doesn't handle non-pagetable aligned hugetlb sizes. Granted,
> that's unlikely at the PUD level, but why be inconsistent..
> 
> So we really want:
> 
> 	if (p*d_leaf(p*d)) {
> 		if (!'special') {
> 			page = p*d_page(p*d);
> 			if (PageHuge(page))
> 				return page_size(compound_head(page));
> 		}
> 		return P*D_SIZE;
> 	}

Still doesn't work because pages can be mapped at funny offsets.

What we really want is for a weak definition of

unsigned long tlb_size(struct mm_struct *mm, unsigned long addr)
{
	if (p*d_leaf(p*d))
		return p*d_size(p*d);
}

then ARM can look at its special bit in the page table to determine
whether this is a singleton or part of a brace of pages.

> Now, when you add !PMD THP sizes (presumably for architectures that have
> 'funny' sizes, otherwise what's the point), then you get to add '||

This is the problem with all the huge page support in Linux today.
It's written by people who work for hardware companies who think only
about exploiting the hardware features they sell.  You all ignore the
very real software overhedas of trying to manage millions of pages.
I see a 6% reduction in kernel overhead when running kernbench using
THPs that may go as large as 256kB.  On x86.  Intel x86, at that.